For data transmission, it has been usual to provide redundancy transmission lines by taking into consideration the possible production of abnormal data caused by faults, and the like on a transmission line. In such a redundancy data transmission device, a transmitter sends data to all of the transmission lines and a receiver receives the data transmitted through any of these transmission lines which are normal.
A means is provided on the side of the receiver for deciding whether or not there is any abnormality of data transmitted from another station through the transmission lines so that, when the decision means determines the existence of abnormality of received data transmitted through a certain transmission line, the receiver switches the transmission line from the line related to abnormal data, to another by means of a changeover switch. When a data abnormality occurs frequently, it is considered as an abnormality of a transmission line and is displayed on a display device such as a CRT or an alert is sounded by actuating an audio device.
When abnormal data is transmitted through a certain transmission line while normal data is transmitted through another transmission line, it is considered that the abnormality occurs not in a transmission circuit of the other station but in a certain transmission. Therefore, an operator is requested to quickly indentify a location of the transmission line where such an abnormal condition exists and to remove the source of the abnormality.
Since, however, the conventional device operates to merely decide whether or not there is an abnormality in the data transmitted through a transmission line, it is difficult for an operator thereof to identify the location of the transmission line in question where the abnormality occurs. Particularly, in a case where a repeater is used to expand the transmission range and area, such identification of the abnormal point becomes more difficult.